In general, the solar cell has the structure shown in FIG. 1. In FIG. 1, a p-type semiconductor substrate 1 is of a plate shape dimensioned 100 to 150 mm squares and 0.1 to 0.3 mm thick, made of polycrystalline or monocrystalline silicon or the like, and doped with a p-type impurity such as boron. An n-type diffusion layer 2 is formed in the substrate by doping an n-type impurity such as phosphorus, and an antireflective coating 3 of silicon nitride (SiN) or the like is formed thereon. A conductive aluminum paste is printed on the back surface by a screen printing technique, before the paste is dried and fired to form a back electrode 6 and a back surface field (BSF) layer 4 at the same time. A conductive silver paste is printed on the front surface, before it is dried and fired to form a collector electrode 5. The solar cell is produced in this way. It is noted that one surface of a substrate which becomes the light-receiving surface of the resulting solar cell is referred to as “front” surface and the other surface of the substrate opposite to the light-receiving surface is referred to as “back” surface.
As mentioned above, for the purpose of reducing any loss of incident light on the front surface, the solar cell includes a thin film of SiN or the like which is formed as an antireflective coating for suppressing surface reflection. It is acknowledged that this thin film forming step serves to form a thin film for suppressing light reflection and is effective for passivating the silicon substrate at the same time. It is known that this passivation capable of reducing the interface state density of silicon substrate is effective for enhancing electrical properties of the solar cell. A thin film formed on the solar cell surface for such purpose is referred to as “passivation” film, hereinafter.
For the purpose of promoting the passivation effect of the antireflective film in the solar cell, efforts are made to develop a CVD apparatus for forming a SiN antireflective film in a polycrystalline silicon solar cell, comprising a CVD chamber where a SiN antireflective film is formed on a polycrystalline silicon substrate, and a heating chamber for maintaining the substrate at the CVD process temperature or heating the substrate at or above the CVD process temperature (see, for example, Patent Document 1: JP-A 2008-306141). The passivation effect promoted by heating by such a method, however, is considered effective for dangling bonds and grain boundary impurities in the bulk of the polycrystalline silicon solar cell. It is thus desired to further enhance the passivation effect.